Investigators now want to know what happened to Caldwell. On Thursday, several tips came in, including one from a man who said Caldwell was his brother. The family has not heard from Caldwell since his disappearance, he said.
The latest clues stem from an investigator determined to identify eight victims of a serial killer, dogged attempts of a family to find a loved one, and a New Jersey trooper tackling a tough case.
"I've been doing a goose chase," said Detective Sgt. Stephen Urbanski of the New Jersey state police, who began investigating the disappearances last year following a tip from law enforcement in Chicago. "This one is really unique."
The boys were among 18 children and four adult staffers from a Paterson, N.J., orphanage on a weeklong trip at the campground, which is surrounded by wilderness. On April 3, 1972, Soden told his sister, April, he was going to run away with Caldwell, but she did not believe he was serious.
She was 13 and had been living at the orphanage with her brother for about a year. Their father had died, and their mother had been struggling to care for them.
Steven and April Soden had three half-siblings who lived with their mother in another state.
In 2011, the Cook County (Ill.) Sheriff's Office launched an effort to identify eight victims whose remains were found in Illinois and were believed to be among 33 victims of serial killer John Wayne Gacy, executed in 1994.
Soden's half-brother, Ronald Soden of Tacoma, Wash., called authorities in Chicago wondering whether Soden had been among the victims there.
Ronald Soden, 72, said that over the last four decades he pursued any lead he thought might help find his brother.
"You never know," he said.
Several relatives provided DNA for the Gacy investigation. Although the DNA did not match Gacy's victims, there was a hit in the national database matching the remains found in 2000 in Burlington County.
A DNA profile had been entered into the University of North Texas Center for Human Identification, which has become a valuable tool for those investigating missing persons, especially older cases, said Angela Williamson of the National Center for Missing and Exploited Children.
"This is really the only avenue we have to identify remains that are so old," Williamson said, adding that once an identification is made, a cause of death may follow. Publicity can help find relatives or witnesses, she said.
"We've been getting leads on Donald's case," Williamson said, declining to elaborate. "It's definitely reinvigorated the whole investigation."
It was only in December that a positive match was made for Soden's remains, dashing the family's hopes that he was still alive.
Urbanski said a new search would be done in the forest to look for additional remains. In the meantime, he had been looking for Caldwell's family.
Although Caldwell had come from a family of seven siblings, finding relatives did not come easily, Urbanski said. The detective found newspaper clippings, unearthed a possible birth certificate, and followed leads that hit dead ends in Florida and other areas.
The orphanage had closed, and its records were destroyed, Urbanski said. He can't find the staffers who were on the camping trip, either.
This week, police and Soden's family discussed the investigation with reporters, hoping publicity would generate new tips.
On Thursday, authorities said they received calls from relatives who saw news reports.
"It's exactly what we were looking for," said Urbanski, who was optimistic but cautioned that investigators still needed to confirm the identity of the relatives and find out what happened to Caldwell.
Anyone with information is asked to call the Center for Missing Children at 1-800-843-5678.
Contact Barbara Boyer at 856-779-3838, firstname.lastname@example.org, or follow on Twitter @BBBoyer.